Marking documents of value such as banknotes is common practice to deter forgery and allow counterfeit banknotes to be readily detected. For example, U.S. Pat. No. 3,858,977 to Baird et al. discloses an optical interference filter having an optical interference layer with a known characteristic of spectral reflectance and a different known characteristic of spectral transmittance, both of which vary with the angle of incident light on the interference filter. The interference filter is disposed on a banknote substrate over a colored portion thereof that is designed to absorb some of the light transmitted by the interference filter so that the interference filer exhibits a color change with a change in the angle of incident light.
U.S. Pat. No. 4,186,943 to Lee discloses a security device for a sheet element such as a banknote. The security device includes a thin film, optically variable element such as a strip or thread disposed within the thickness of the sheet element. The thin film element is in the form of a dichroic filter having a known spectral reflectance and transmittance. The sheet element has a pair of superposed windows between which the thin film element extends so as to be visible through each window. The dichroic filter includes a transparent Melinex substrate coated with a seven-layer stack of alternate high and low refractive index materials. Layers of high refractive index are disposed adjacent the sheet element and the top of the stack. The layers of high refractive index have a thickness equal to ¾ wavelength and have an index of refraction equal to 2.3. The layers of low refractive index have a thickness equal to ¼ wavelength and have an index of refraction equal to 1.55. This combination yields a thin film element that reflects green and transmits in magenta when viewed normally.
Unfortunately, consistently producing seven-layer dichroic filters of this nature in large numbers is a very difficult task, especially given the very tight color specifications required for use in banknotes. As will be appreciated, banknotes are produced in extremely large numbers (i.e. millions and in fact sometimes billions) and must be identical in order to avoid public confusion, which makes counterfeiting easier. In addition, the fabrication cost associated with producing the seven-layer dichroic filters is high, translating into a high banknote unit cost.
An alternative security device for banknotes including a metal-dielectric stack has been developed by Agra Vadeko Inc. of Oakville, Ontario, Canada, assignee of the present invention in collaboration with the Bank of Canada and is sold by De La Rue under the name Colorshift™. The security device is designed for use as a windowed security feature incorporated within the banknote. The security device exhibits a color change from magenta to green with a change in view angle and has been featured in over 4 billion banknotes worldwide. The metal-dielectric stack includes a polyethylene teraphathalate (PET) substrate with an Al/SiO2/Inconel® multilayer coating on the substrate. The aluminum layer is disposed on the PET substrate and is demetallized to include images, patterns and/or text.
Although this security device is highly satisfactory for the protection of banknotes, because the aluminum layer is sandwiched between the PET substrate and the SiO2 layer, the aluminum layer must be demetallized before the manufacturing process of the security device. This requires the end user of the security device i.e. the issuer of the banknotes, to disclose highly sensitive information to the security device manufacturer. In addition, since the aluminum layer must be demetallized in a separate process to include the images, patterns and/or text before the security device can be completed, longer lead times for security device manufacturers are required due to the fact that the security device manufacturers must wait for the images, patterns and/or text to be included on the aluminum layer, which are specific to a given customer and banknote denomination. This of course makes it virtually impossible for security device manufacturers to stockpile material and reduce manufacture lead times.
It is therefore an object of the present invention to provide a novel secure document of value including a thin film, optically variable security device and to a method of manufacturing the same.